Pressure sensors, usually in the form of a transducer, are widely used in a variety of fields. Pressure transducers are used in sensors for automotive applications to measure, among other things, oil pressure, water pressure, and manifold absolute pressure. Pressure transducers can be assembled in a variety of configurations, but generally involve some form of piezo element.
A piezo element can be a piezoresistive element or a piezoelectric element. A piezoresistive element is an element that produces a change in resistance in response to an applied force. In contrast, a piezoelectric element produces an electrical current in response to an applied force. Either type of element can be used to generate a signal proportional to the applied force by measuring the change in resistance or voltage, as applicable. In a piezoresistive element, or piezoresistor, for example, as the force is applied the resistance of the element changes, which produces a proportional change in a reference voltage applied to the piezoresistor. The force applied can then be calculated from this change in the reference voltage.
In one configuration, a pressure transducer can comprise a flexible diaphragm fitted with one or more piezo elements. When pressure is applied to the diaphragm, the diaphragm deflects. This deflection, in turn, places the piezo elements mounted thereon under compression or tension, depending on their location on the diaphragm, effectively measuring the deflection of the diaphragm. This method can be used to measure the difference between two pressures applied to opposite sides of the diaphragm (differential pressure) or an applied pressure on one side and a reference pressure on the other side (absolute pressure).
This type of sensor does not compensate for outside forces (i.e., forces other than pressure) acting on the diaphragm. When used in a jet engine, for example, the sensor can be exposed to significant heat, vibration, and acceleration. A portion of the change in the piezo elements, therefore, can be caused by these outside forces resulting in inaccurate measurements.
One solution to this problem, as illustrated in U.S. Pat. No. 6,293,154 (“the '154 patent”), assigned to Kulite Semiconductor Products, Inc. (the Applicant herein) is to provide a correction for this error using a second, hermetically sealed diaphragm that is substantially colocated with the differential or absolute pressure diaphragm. The second diaphragm is enclosed in a hermetically sealed chamber with equal pressure on both sides of the diaphragm. Piezoresistors are fitted to both diaphragms to measure their deflection.
In this configuration, the deflection of the second diaphragm due to pressure is substantially zero and any change in resistance in the piezoresistors mounted thereon is the result of, for example, vibration, heat, and/or hysteresis only (i.e., “non-pressure effects”). The signal from the second diaphragm can be subtracted from the signal from the first diaphragm, which inherently includes changes in resistance due to both pressure and non-pressure effects, producing a signal that is proportional only to pressure.
When the pressure sensor is an absolute pressure sensor the method set forth in the '154 patent works very well because the cavity under both diaphragms is at a sealed reference pressure. However, when the sensor is to be a differential or gauge sensor, it is necessary to expose the back side of the pressure sensitive diaphragm to a second pressure while still maintaining a sealed reference pressure on the non-pressure sensitive diaphragm. This sealed pressure is necessary on the non-pressure sensitive diaphragm so that it does not respond to any outside change in pressure. Conventionally, the piezoresistors mounted on the first diaphragm and the piezoresistors mounted on the second diaphragm were internally connected and shared common terminals. As explained in more detail below, these common connections make it difficult to achieve and maintain a hermetic seal on the chip.